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Abstract:

A laser system comprising: a light source generating light, said light
source comprising at least two laser sources of different wavelengths;
and a frequency converter operatively coupled to said light source to
accept the light provided by said light source and to convert it to
higher optical frequency such that said frequency converter is producing
light output at the final output wavelength situated in the 150-775 nm
range.

Claims:

1. A laser system comprising:a light source generating light, said light
source comprising at least two laser sources of different wavelengths;
anda frequency converter operatively coupled to said light source to
accept the light provided by said light source and to convert it to
higher optical frequency such that said frequency converter is producing
light output at the final output wavelength situated in the 150-775 nm
range.

2. The laser system of claim 1, wherein said laser system does not include
an Optical Parametric Oscillator (OPO).

3. A laser system comprising:a light source generating a light, said light
source comprising at least two fiber laser sources of different
wavelengths, each providing more than 10 W of optical power; anda
frequency converter operatively coupled to said light source to accept
the light provided by said light source and to convert it to higher
optical frequency such that said frequency converter is producing light
output at the final output wavelength situated in the 150-775 nm range.

4. The laser system of claim 2 wherein each of said at least two fiber
laser provides more than 50 W of optical power.

5. The laser system of claim 2 wherein at least one of said two fiber
laser sources is tunable.

6. The laser system of claim 2, wherein said light source is a pulsed
light source and the light provided to said frequency converter is pulsed
light.

7. The laser system of claim 7, wherein the pulsed light source has a
pulse width of 0.01 to 100 ns and a duty cycle of 1:2 to 1:1000000.

8. The laser system of claim 7, wherein pulsed light source is a master
oscillator power amplifier MOPA.

9. The laser system of claim 1, wherein one of at least one of said two
laser sources is a Yb doped fiber laser or fiber amplifier.

10. The laser system of claim 10, wherein the other one of said two laser
sources is Nd doped fiber laser or fiber amplifier.

11. The laser system of claim 10, wherein the other one of said two fiber
lasers is Er doped fiber laser.

12. The laser system of claim 1, wherein said at least two laser sources
are arranged in a parallel configuration so as to provide the frequency
converter with two output wavelengths.

13. The laser system of claim 1, wherein said at least two laser sources
are arranged in a parallel configuration so as to provide the frequency
converter with two synchronous output wavelengths.

14. The laser system of claim 1, wherein the pulsed light source comprises
a tunable laser for tuning the source wavelength, wherein the tuning of
the source wavelength provides fine tuning of the final output
wavelength.

15. The laser system of claim 1, wherein said frequency converter includes
no more than 4 conversion crystals.

16. The laser system of claim 8, further comprising a high-power optical
fiber amplifier for amplifying the pulsed light to increase and set the
peak pulse power, wherein the high-power optical amplifier comprises an
optical fiber doped with at least one rare-earth dopant member selected
from a group consisting of Ytterbium, Erbium, and Thulium.

[0005]Coherent light sources in the visible (400-775 nm) wavelength range
and in the UV or deep UV (DUV) range (150-400 nm) find a number of
important applications (such as in medicine, life sciences material
processing, photolithography and metrology). Typically, a high output
power is desired and different output wavelengths are required for
different applications.

[0006]However, in contrast to the widely available light sources developed
for the near-IR spectral ranges, the choice of the shorter wavelength
light sources (e.g., visible or UV) is very limited. Excimer lasers are
often utilized to produce UV radiation at 248 nm, 193 mm, and 157 nm.
However, these lasers are expensive, costly to maintain, have relatively
poor beam quality, and are not tunable.

[0007]Harmonic conversion in nonlinear crystals is typically used to
convert the IR (infrared) wavelength output of the diode pumped solid
state (DPSS) laser to UV and visible ranges. Unfortunately, only a few
discrete wavelengths are available from DPSS lasers, and therefore, the
output wavelengths that are produced by this method are also limited to
harmonics (e.g., 2nd, 3rd, 4th) of the fundamental or pump
wavelengths. Such laser outputs are, for example, 532 nm, 355 nm, and 266
nm that are produced by harmonic conversion of 1064 nm Nd:YAG laser
output.

[0008]Optical Parametric Oscillators (OPO) may be utilized with a DPSS
laser to provide additional output wavelength tunability, provided that a
nonlinear crystal with a suitable transparency range and phase matching
conditions exists. This is not always possible. Furthermore, because the
output wavelength from OPO is determined by phase matching conditions of
the nonlinear crystal, the laser systems utilizing OPOs are generally
more complex, and suffer from poor stability, as compared to the laser
systems that utilize harmonic converters only.

[0009]An additional disadvantage of DPSS lasers is that the average power
output is limited to relatively low (10-25 W) values by thermal issues
(heat dissipation in the laser crystal). To achieve high peak optical
power values required for efficient nonlinear frequency conversion, they
are typically operated either in Q-switched (long, 30-50 ns pulses)
regime where the pulse repetition frequencies are limited to several kHz,
or in a mode-locked (5-10 ps pulses) regime where the spectral width of
the output is significantly larger, and therefore coherence length of the
laser output is shorter then that of a continuous wave or CW laser.
Therefore, such DPSS lasers are not suitable for producing quasi-CW
output, where optical pulses are sufficiently long to keep the high
coherence, but at the same time repetition frequency is high enough so
that for a particular detector the output light appears effectively CW.

[0010]Therefore, a need still exists to develop high power, efficient and
stable quasi-CW laser sources in the 0.15-0.775 μm range.

SUMMARY OF THE INVENTION

[0011]One aspect of the invention is a laser system comprising: (i) a
light source generating light, said light source comprising at least two
laser sources of different wavelengths; and (ii) a frequency converter
operatively coupled to said light source to accept the light provided by
said light source and to convert it to higher optical frequency such that
said frequency converter is producing light output at the final output
wavelength situated in the 150-775 nm range. Preferably the two laser
sources are fiber lasers or seeded fiber amplifiers.

[0012]Additional features and advantages of the invention will be set
forth in the detailed description which follows, and in part will be
readily apparent to those skilled in the art from that description or
recognized by practicing the invention as described herein, including the
detailed description which follows, the claims, as well as the appended
drawings.

[0013]It is to be understood that both the foregoing general description
and the following detailed description present embodiments of the
invention, and are intended to provide an overview or framework for
understanding the nature and character of the invention as it is claimed.
The accompanying drawings are included to provide a further understanding
of the invention, and are incorporated into and constitute a part of this
specification. The drawings illustrate various embodiments of the
invention and together with the description serve to explain the
principles and operations of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1A illustrates the ranges of the output wavelengths that can be
generated by harmonic conversion using only Yb-doped fiber laser source,
only Er-doped fiber laser source, and both Yb- and Er-doped fiber laser
sources.

[0015]FIG. 1 is a block diagram view of the laser system 10 according to
one embodiment of the present invention;

[0016]FIG. 2 illustrates schematically second exemplary embodiment of the
laser system 10 according to the present invention;

[0017]FIG. 3 illustrates schematically third exemplary embodiment of the
laser system 10 according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018]A non-cavity or non-resonant method and apparatus for generating
coherent light are taught herein. In accordance with some embodiments of
the present invention, a pulsed light source comprising at least two
light sources of different wavelengths is used in the inventive method
and apparatus to provide light to the frequency converter that converts
it to higher optical frequency such that the frequency converter produces
light output at the final output wavelength situated in the 150-775 nm
range. The at least two light sources can be either lasers or seeded
optical amplifiers (laser amplifiers), or a combination thereof and they
are refereed to as laser sources or lasers herein.

[0019]Reference will now be made in detail to the present preferred
embodiments of the invention, examples of which are illustrated in the
accompanying drawings. Whenever possible, the same reference numerals
will be used throughout the drawings to refer to the same or like parts.
One embodiment of the laser system of the present invention is shown in
FIG. 1, and is designated generally throughout by the reference numeral
10.

[0020]Referring to FIG. 1, a laser system 10 of this embodiment includes a
light source 102 that comprises two "master oscillator-power amplifiers"
(MOPAs) that operate in parallel to simultaneously provide first pulsed
light outputs 108A and 108B. While in principle, the laser system 10 may
be a CW system, in this embodiment, it utilizes an optional electrical
pulse generator 104A driving optical modulators 104B and 104C to provide
pulse modulations of the master oscillators' (MOPAs') light.

[0021]In this embodiment, the light from the 1105 nm seed source 112
passes through the optical modulator 104B driven from the electrical
pulse generator 104A. The modulated pulsed light enters the fiber
amplifier 106A comprising an Yb-doped silica based fiber 106A', and the
amplifier 106A generates amplified pulsed light output signal 108A having
an optical spectrum centered at an output wavelength
λ1Aout=1105 nm. The light from the 1550 nm seed source 114
passes through the optical modulator 104C and enters the amplifier 106B,
for example an Er-doped fiber amplifier. The amplifier 106B provides
amplified pulsed light output signal 108B having an optical spectrum
centered at an output wavelength λ1Bout=1550 nm. Thus, in this
embodiment, since the optical modulators 104B and 104C are driven from
the same electrical pulse generator 104A, the pulsed light source 102 of
the laser system 10 provides synchronized light pulses 108A, 108B at two
different wavelengths. The seed sources, as well as optional pulse
generator (s), modulators and/or optical delay element(s) D comprise the
initial (pulse) source 102'. That provide (two different wavelength)
light to high power lasers or/and amplifiers 106A, 106B.

[0022]The frequency converter 110 is operatively coupled to the light
source 102 (which in this embodiment is a pulsed light source) to accept
the first pulsed light output 108A, 108B at the respective wavelengths
λ1Aout, λ1Bout, and to convert it to higher optical
frequency, such that the frequency converter 110 is producing the final
pulsed light output 112 at the wavelength λout situated in the
150-775 nm range. The frequency converter 110 may include second harmonic
generation (SHG) and sum frequency mixing (SFM) stages. The simplest type
of a frequency converter is a harmonic generator producing, for example,
2nd, 3rd, or 4th harmonic, etc., which means that it is
converting the first output wavelength λ1Aout to the final
wavelength of λout=λ1Aout/2; λ1Aout/3,
or λ1Aout/4. In general for arbitrary combination of an
arbitrary number of SHG and SFM stages,
1/λout=m/λ1Aout+n/λ1Bout, where m and n
are integer numbers.

[0023]According to some of the embodiments, the pulse width provided by
the light source 102 is 0.01 to 100 ns and a duty cycle of the pulses is
1:2 to 1:1000000, for example 0.1 ns to 10 ns with duty pulse cycle of
1:3 to 1:1000. For optimum efficiency, it is important to consider the
requirements for the optical pulse width and repetition frequency. In
principle, there is no upper limit on the optical pulse width. However,
for pulses longer than a few nanoseconds, stimulated Brillouin scattering
(SBS) in the amplifier (or laser) fiber can limit the maximum amount of
power that can be converted and one has to make sure that the pump
optical spectrum is broad enough to suppress SBS. Also, to increase the
nonlinear conversion efficiency in crystals, it is desirable that the
duty cycle (the ratio of pulse width and repetition period, which is also
the ratio of average to peak power) is less than 1:100, which, for pulses
longer than 10 ns, will limit the repetition frequency to values lower
than 1 MHz, which is not desirable if the goal is to produce a quasi-CW
source.

[0024]The high-power optical amplifiers 106A, 106B amplify the pulsed
light 104 from the seeds 112 and 114, such that the average power and the
peak pulse power of the pulsed light source 102 can be increased. In this
way, cost-effective pump sources based on the well developed fiber
amplifier technology for the amplifiers 106A, 106B may be utilized. The
method and apparatus of the present invention are especially suitable for
use with Yb-doped or Er-doped fiber optical amplifiers, but can also be
used with other types of power amplifiers 106A, 106B. More specifically,
it is noted that an Yb-doped fiber based laser can provide an optical
output in the 1030 to 1120 nm range and Er, Tm and Nd-doped silica fiber
based lasers or amplifiers are capable of providing an output in
1530-1610 nm, 1800-2000 nm, and 890-930 nm ranges, respectively.
Adjustment in the output wavelength of these diode pumped fiber lasers
will allow one to adjust/tune the final output wavelength to its desired
value. In addition, due to long (meters) length of the active medium,
fiber lasers do not suffer from heat dissipation issues as much as DPSS
lasers and are therefore capable of providing much higher average power
output, keeping a perfect single transverse mode beam quality. Thus,
fiber lasers are perfect candidates for creating high power CW, quasi-CW
or nanosecond pulse sources in visible and UV ranges by harmonic
conversion.

[0025]FIG. 1A illustrates the wavelength ranges that can be generated from
an Yb-doped fiber laser source and its harmonics (top), Er-doped fiber
laser source and its harmonics (middle), and these two fiber lasers
sources together (bottom), including the shorter wavelengths produced by
SHG and SFM. FIG. 1A illustrates that using both of these two laser
sources one can achieve an output at a wider variety of wavelengths,
including those ranging from less then 200 nm to 400 nm, and several
ranges of longer wavelengths. The advantage is evident in FIG. 1A in that
the bottom plot contains wavelength bands not present in either of the
two top plots (which means that those can be generated only when using
both Yb- and Er-doped fiber lasers). Different combination of laser
sources (e.g., Er and Nd; Yb and Tm; Er and Tm) would, of course, allow
one to produce the output in different wavelength ranges, some of which
may overlap with those depicted in FIG. 1A.

[0026]According to some embodiments of the present invention the light
source 102 includes a tunable laser for tuning the source wavelength
λp, wherein the tuning of the source wavelength (and harmonic
conversion stages, if required) provides fine tuning of the final output
wavelength λout.

[0027]Due to a very long (several ms) lifetime of the excited states,
rare-earth (e.g., Er or Yb) doped fiber amplifiers essentially amplify
the average power of the incoming signal, and for a very small duty
cycle, an amplifier 106A, 106B with only a modest average power output
can produce very large peak pulse power. For example, 1 ns long pulses
from master oscillators (seed 112 and pulse modulator 104C; seed 114 and
pulse modulator 104B such as, for example, externally modulated
distributed feedback (DFB) laser diodes) can be amplified to a peak power
of 20 kW in a multiple-stage Er and Yb-doped (fiber) amplifiers 106A,
106B, while the average output power of power amplifiers 106A, 106B is
only 2 W, if the repetition rate is 100 KHz (peak power is 10000×
average power).

[0028]Directly modulating a semiconductor laser diode with an electrical
pulse generator or connecting the diode output, as described above, to a
separate electro-optic intensity modulator for setting the pulse width,
can be used to make the initial pulsed light source 102' for generating
the pulsed light 104, with or without further amplification (i.e. with or
without the amplifiers 106A, 106B). As noted above, forming, a
rectangular pulse is preferred for maximizing the frequency conversion
efficiency (minimizing the effect of incomplete conversion in the pulse
wings) and minimizing spectral broadening in high-power fiber amplifiers
(by self-phase modulation (SPM)). Since Er, Yb, Tm and Nm-doped
amplifiers 106A, 106B have a relatively wide spectral gain bandwidth
(several 10s of nanometers), the pulsed light source 102 and to some
extent the whole laser system 10 can be made wavelength tunable or
adjustable by using a tunable master oscillator pulse source 102' (such
as an external cavity semiconductor laser, directly modulated or coupled
to a separate modulator).

[0029]As already mentioned above, output wavelength range of approximately
1030-1120 nm is directly accessible for an Yb-doped fiber laser systems.
Other types of fiber laser systems, such as those including a silica
based fiber which is Er--Yb co-doped for 1530-1570 nm range, Nd-doped for
890-930 nm range (working at the 3-level transition) or Tm-doped for
1800-2000 nm range, can also be utilized. Frequency converter 110 can
include a number of stages, each one generating a second, third or fourth
harmonic or performing a sum frequency mixing of the fiber lasers and
preceding stages outputs, to provide the desirable output wavelength at
the end. Output wavelengths λout can be produced by sum
frequency mixing, in a suitable nonlinear crystal, the outputs of two
different fiber lasers or amplifiers. Since the two lasers/seeded
amplifiers 106A, 106B provide different output wavelengths and are
tunable or adjustable within a range of wavelengths, using the two of
such lasers/amplifiers in conjunction with one another greatly enhances
our ability to tune or adjust the final output wavelength λout.
Therefore, in accordance with the teachings of the present invention, any
desired output wavelength λout in the 150-775 nm range can be
produced by a suitable combination of two pulsed fiber lasers or seeded
amplifiers 106A, 106B, and the frequency converter 110 that includes
harmonic generation stage(s) and/or sum-frequency mixing stage(s), as
will be illustrated, for example, below.

[0030]In the following, we present examples of laser systems 10 producing
some specific output wavelengths, of interest for specific applications.
In the following exemplary embodiments, we choose to design frequency
converters 110 in that utilize exclusively borate nonlinear crystals
(LBO, BBO, CLBO), which are known to have the highest optical damage
thresholds and are therefore capable of producing higher powers by
harmonic conversion. Advantageously, these laser systems 10 achieve
relatively high conversion efficiency, while avoiding or minimizing
potential crystal damage caused by incident high power beams at short
wavelengths (UV). This is done, at least partially, by "trading" short
wavelength (UV) power for long wavelength (IR) power when doing sum
frequency generation (SFG) to produce UV output wavelength (since the
output power of the SFG stage is proportional to the product of the two
input powers, more of the IR light and less of the UV can be supplied to
the input of the stage to produce the same output). Those skilled in the
art will appreciate that these examples represent only a small subset of
the many possibilities and that other non-linear crystals may also be
utilized.

Example 1

Laser system I for Producing an Output at λ=193.0 nm

[0031]Sub-200 nm laser light sources are very important for metrology
applications in the semi-conductor industry. As the feature sizes of
integrated circuits are shrinking, shorter wave-length light is used for
a photolithography. Mask and wafer inspection, as well as optics
manufacturing is then in need of the same or similar DUV light
wavelength. The systems presently used, based on solid-state laser
sources, harmonic conversion and OPOs, typically work at very low
repetition rates, are very bulky, complex, expensive and require frequent
and complicated maintenance.

[0032]FIG. 1 illustrates schematically the first exemplary embodiment of
the 193.0 nm laser system 10. As described above, the optical system 10,
according to this embodiment, includes two optical fiber amplifiers 106A,
106B that provide synchronized pulse outputs of different wavelengths to
the frequency converter 110. In this exemplary embodiment, the Yb-doped
fiber amplifier 106A of the light source 102, produces a narrow linewidth
output centered at the wavelength λ1Aout=1104 nm. The first
output signal 108A from the amplifier 106A is then provided to the first
stage of the frequency converter 110. In this embodiment, the frequency
converter 110 includes two LBO (Lithium triborate, LiB3O5)
crystals 110A, 110B, a BBO (beta barium borate, β-BaB2O4)
crystal 110C, and a CLBO (cesium lithium borate, CsLiB6O10)
crystal 110D. The three nonlinear crystals, LBO 110A, LBO 110B and BBO
110C, are used to generate the 5th harmonic of the 1104 nm
wavelength by: (i) second harmonic generation (SHG) via LBO 110A
producing wavelength of 552 nm, (ii) a third harmonic generation via
sum-frequency mixing (SFM) of residual 1104 nm light and 552 nm light
within the LBO 110B, producing the 368 nm wavelength, (iii) sum-frequency
mixing (SFM) of 368 nm and 552 nm light via BBO 110C producing 220.8 nm
output. The LBO crystal 110B receives the light at 552 nm and converts
part of it (1 to 90%, preferably 50%) to 368 nm light. In this
embodiment, a custom waveplate WP is needed between the two LBO crystals
110A and 110B to rotate one of the polarization states (of 1104 nm light
or 552 nm light) but not the other, so that they are aligned along the
same direction at the second LBO crystal 110B. Any residual light at 1104
nm wavelength is filtered out of the system by filter (dichroic mirror)
M1. This 368 nm light, exiting the LBO crystal 110B together with the
residual 552 nm light (99% to 10%), is then provided to the BBO crystal
110C which generates, via sum frequency mixing (SFM), light at the
wavelength of 220.8 nm. The Er-doped fiber amplifier 106B of the light
source 102, produces a narrow linewidth output centered at the wavelength
λ1Bout=1535 nm. The first output signal 108B from the
amplifier 106B is then provided to the stage 110D of frequency converter
110 which, in this embodiment, is the CLBO crystal. Any residual light at
552 nm or 368 nm wavelengths is filtered out of the system by filter
(dichroic mirror) M2. The first output signal 108B at the (IR) wavelength
of 1535 nm is then sum frequency mixed within the fourth stage 110D (CLBO
crystal) with the 220.8 nm light provided by the BBO crystal 110C, so
that the fourth stage 110D (CLBO crystal) produces the output at the
wavelength of λout=193.0 nm. The output power Pout of the
output wavelength λout=193.0 nm is proportional to the product
of the two input powers Pout˜P1535 nm×P220.8
nm, where P1535 nm is the optical power provided to the CLBO crystal
from the amplifier 106B (IR wavelength, 1535 nm) and P220.8 nm is
the optical power provided to the CLBO crystal from the BBO crystal 110C
(UV wavelength, 220.8 nm). Because the damage to the nonlinear crystals
is primarily caused by the high power beams in short wavelengths range,
we can provide less optical power from the BBO crystal 110C, and more
power from longer wavelength source (the amplifier 106B, P1535 nm),
thereby "trading" short wavelength (UV) incident power for long
wavelength (IR) incident power, and thus avoiding or minimizing potential
crystal damage caused by incident high power beam at short wavelength
(UV). Accordingly, it is preferable that laser that provides longer
wavelength light to the last stage of the frequency converter 110 (or to
any SFM stage), such as Er-doped fiber amplifier 106B, provide the output
optical power of at least 10 W, and preferably at least 50 W.

[0033]The optimum temperature for the first LBO crystal 110A (as predicted
by SNLO, a free nonlinear crystal modeling software package from Sandia
National Laboratories) is 376.4 Kelvin. At this temperature, the crystal
operates in non-critical phase matching (for light propagating at the
angles of θ=90° and φ=0° to the optic axes of the
crystal--LBO is a so-called bi-axial crystal) with the effective
nonlinearity coefficient of deff=0.85 pm/V and essentially zero
birefringent walk-off for the second harmonic generation of 552 nm. The
second LBO crystal 110B can not be non-critically phase matched. For the
light propagating at θ=90° and φ=32.6° to the
optic axes, the phase matching temperature for the sum frequency mixing
of 1104 and 552 nm is 433 K, the effective nonlinearity is deff=0.75
pm/V and the birefringent walk-off is 15.99 milliradians. The BBO crystal
110C is a uni-axial crystal. For light propagating at
θ=64.2° to its optic axis, the phase-matching crystal
temperature is 433 K, the effective nonlinearity is deff=1.3 pm/V
and the birefringent walk-off is 71 milliradians, for the nonlinear
process of sum frequency mixing of 552 nm and 368 nm light. The fourth
crystal, CLBO is a uni-axial crystal. For light propagating at
θ=62.3° to its optic axis, the phase-matching crystal
temperature is 433 K, the effective nonlinearity is deff=1.01 pm/V
and the birefringent walk-off is 37.33 milliradians. As shown in FIG. 1,
no OPOs were utilized in his embodiment of the laser system 10. Table I
provides the summary of crystal's parameters utilized in the laser system
10 of example 1.

[0034]In Table I, as well as in all subsequent examples, the first row
lists the type of the nonlinear crystal(s) used and the second the type
of a nonlinear process the crystal is performing. Rows 3-5 list the
output and two input wavelengths (for the case when the nonlinear process
is a second harmonic generation, the two input wavelengths are the same).
Row 6 provides the crystal temperature and rows 7-8 provide the
propagation direction angles with respect to the crystal optic axes
required for phase matching. Row 9 specifies the effective nonlinearity
coefficient (a measure of how efficient the conversion can be for a given
input power and crystal length), and row 10 provides the value for input
and output beam angular walk-off (slight angular separation of the input
light and the harmonic light within the crystal) caused by crystal
birefringence.

[0035]The non-critical phase matching (NCPM) at θ=90° (and
φ=0°, for bi-axial crystals) is the most preferable kind since
it allows maximum angular and spectral acceptance (deviation of
propagation direction and wavelength allowable without significant
degradation of the conversion efficiency) for second harmonic generation
and is characterized by zero or nearly zero birefringent walk-off of the
pump and second harmonic light beams, which allows using long crystals to
achieve high conversion efficiency with moderate levels of peak power.

[0036]The advantage of the example laser system 10 of FIG. 1 is that it
provides the sub-200 nm output, starting with Er- and Yb-doped fiber
lasers for which a well developed manufacturing technology is available.
However, it exhibits a significant birefringent walk-off (71.6 mrads) in
the BBO crystal 110C. Large walk-off does not allow tight focusing of the
laser beams and therefore results in the lower conversion efficiency,
since a shorter crystal or larger beams (lower optical power density)
have to be used. The walk-off influence can be reduced if multiple
180° rotated crystals of the same kind are used, but this is
likely to reduce the useful lifetime of the device, because more surfaces
will be exposed to the high optical power. Diffusion or adhesive-free
bonding can be utilized to eliminate additional exposed crystal surfaces
by seamlessly joining the 180° rotated crystals together. Another
possible solution is to focus the incoming light beam(s) into an
elliptical spot within the nonlinear crystal, with the longer axis of the
ellipse oriented along and the shorter axis perpendicular to the walk-off
direction. In this case, the higher conversion efficiency can be achieved
(due to the tighter focusing in the no walk-off direction and therefore
higher power density and the possibility to use a longer crystal) while
at the same time minimizing a beam distortion caused by the walk-off.

Example 2

Laser System for Producing an Output at λ=193.4

[0037]FIG. 2 presents an example of a 193.0 nm laser system 10 according
to another embodiment of the present invention. The laser system 10 of
this embodiment is similar to that of the embodiment of example 1 in that
it includes a pulsed light source 102 comprising two seeded high power
optical amplifiers 106A, 106B that (in parallel) provide synchronized
first pulsed light 108A, 108B to the frequency converter 110. However, in
this embodiment the first amplifier 106A is Nd-doped (SiO2 based)
fiber amplifier. More specifically, the 935.6 nm output of Nd-doped
amplifier 106A is provided to the first stage 110A. (LBO crystal) of the
frequency converter 110. The 1104 nm output of Yb-doped fiber amplifier
106B is simultaneously provided to the CLBO crystal (3rd stage 110C
of the frequency converter 110). The frequency converter 110 includes one
LBO crystal 110A, one BBO crystal 110B and one CLBO crystal 110C. The
three nonlinear crystals, LBO 110A, BBO 110B and CLBO 110C, are used to
generate 193.0 nm wavelength by: (i) second harmonic generation (SHG) via
LBO 110A producing wavelength of 467.8 nm, (ii) another SHG (BBO 110B
producing the 233.9 nm wavelength), (iii) and sum-frequency mixing (SFM)
of 233.9 nm provided by the BBO crystal 110B and the 1104 nm light
provided by the Yb-doped amplifier 106B, via CLBO 110C, producing 193.0
nm output. More specifically, LBO and BBO crystals 110A and 110B are
second harmonic generators (SHGs). The LBO crystal 110A receives the
first output wavelength λ1out of 935.6 nm from the Nd-doped
fiber amplifier 106A and provides 467.8 nm output to the second BBO
crystal 110B. Any residual light at 935.6 nm wavelength is optionally
filtered out of the system by filter such as a dichroic mirror (not
shown). The BBO crystal 110B receives the light at 467.8 nm and converts
part of it to the 233.9 nm light. The remaining 467.8 nm light is then
optionally filtered out by the dichroic mirror M1. The 233.9 nm light,
exiting the BBO crystal 110B, together with the 1104 nm light from the Yb
doped fiber amplifier 106B, is then provided to the CLBO crystal 110C
which generates, via sum frequency mixing (SFM), light at the desired
wavelength λout=193.0 nm. The operating temperature for the
first LBO crystal 110A is 433 Kelvin. The phase matching angles θ
and φ are 90° and 16.4°, respectively. The crystal
operates with the effective nonlinearity coefficient of deff=0.83
pm/V and only 9.28 mrad of birefringent walk-off for the second harmonic
generation of 935.6 nm. For the second crystal, BBO 110B, for the light
propagating at φ=59.0° to the optical axis, the phase matching
temperature for the second harmonic generation of 233.9 nm is also 433 K,
the effective nonlinearity is deff=1.45 pm/V and the birefringent
walk-off is 78.3 milliradians. The third crystal, CLBO 110C, operates in
non-critical phase matching (for light propagating at the angle of
θ=90° to the optical axis of the crystal) with the effective
nonlinearity coefficient of deff=1.12 pm/V and essentially zero
birefringent walk-off. Optimum operating temperature for this crystal is
384 K for the nonlinear process of sum frequency mixing of 233.9 and 1104
nm. The output power Pout of the output wavelength
λout=193.0 nm is proportional to the product of the two input
powers Pout˜P1104 nm×P233.9 nm, where
P1104 nm is the optical power provided to the CLBO crystal from the
amplifier 106B (IR wavelength, 1104 nm) and P233.9 nm is the optical
power provided to the CLBO crystal from the second stage BBO crystal 110B
(LTV wavelength, 233.9 nm). Because the damage to the nonlinear crystals
is primarily caused by the high power beams in short wavelengths range,
we can provide less optical power from the BBO crystal 110C, and more
power from longer wavelength source (the amplifier 1068, P1150 nm),
thereby "trading" short wavelength (UV) incident power (on the crystal)
for long wavelength (IR) incident power, and thus avoiding or minimizing
potential crystal damage caused by incident high power beams in short
wavelengths (UV). Accordingly, it is preferable that laser that provides
longer wavelength light to the last stage of the frequency converter 110
(or to any SFM stage), such as Yb-doped fiber amplifier 106B, provide the
output optical power of at least 10 W, and preferably at least 50 W.

[0038]Table II provides the summary of crystal's parameters utilized in
the laser system 10 of example 2.

[0039]For simplicity, additional optical elements are not shown in optical
schematics given for the examples. Those skilled in the art will be able
to determine where and when such elements should be used. These optional
elements are, for example, lenses for focusing light beams on the
nonlinear crystals, to increase conversion efficiency, waveplates used to
rotate the polarization of light, additional dichroic mirrors, beam
splitters etc.

[0040]The advantage of the laser system 10 of FIG. 2 is that a minimum
number of nonlinear crystals (only 3) are used to produce the sub-200 nm
output. However, it exhibits a significant birefringent walk-off (78
mrad) in the BBO crystal 110B. Large walk-off does not allow tight
focusing of the laser beams and therefore results in the lower conversion
efficiency, since a shorter crystal or larger beams (lower optical power
density) have to be used. The walk-off influence can be reduced if
multiple 180° rotated crystals of the same kind are used, but is
likely to reduce the useful lifetime of the device, because more surfaces
will be exposed to the high optical power. Diffusion or adhesive-free
bonding can be utilized to eliminate additional exposed crystal surfaces
by seamlessly joining the 180° rotated crystals together. Another
possible solution is to focus the incoming light beam(s) into an
elliptical spot within the nonlinear crystal, with the longer axis of the
ellipse oriented along and the shorter axis perpendicular to the walk-off
direction. In this case, the higher conversion efficiency can be achieved
(due to the tighter focusing in the no walk-off direction and therefore
higher power density and the possibility to use a longer crystal) while
at the same time minimizing a beam distortion caused by the walk-off.

[0041]Those skilled in the art will appreciate that in this and other
examples, even with the exact same set of wavelengths, different
nonlinear crystals can be used to perform the required conversion.

[0042]The last crystal (CLBO) is close to the non-critical phase matching
condition and therefore, birefringent walk-off is nearly negligible. In
addition, a high peak IR (1104 um) power is supplied to it directly from
Yb-doped MOPA. This can result in conversion efficiency in respect to UV
power approaching 80%, and therefore minimum incoming UV power into the
CLBO crystal will be needed to achieve the same DUV (deep UV) power
output, thus minimizing optical damage to the CLBO crystal. It is noted
that the optical power values as well as temperatures, phase matching
angles, effective nonlinearity coefficient and birefringent walk-off
values shown in Table II (and other Tables provided herein) are given
only as a guideline. Other configurations and operating temperatures may
also be utilized.

Example 3

Laser System for Producing an Output at λ=198.7 nm

[0043]FIG. 3 illustrates schematically the exemplary embodiment of the
198.7 nm laser system 10. As described above, the optical system 10
according to this embodiment, includes two seeded optical fiber
amplifiers 106A, 106B that provide synchronized pulsed outputs of
different wavelengths to the frequency converter 110. In this exemplary
embodiment, the 1064 nm seeded Yb-doped fiber amplifier 106A of the light
source 102, produces a narrow linewidth output at the wavelength
λ1Aout=1064 nm. Seeded Er-doped fiber amplifier 106B
simultaneously produces light 108B at a narrow linewidth output at
wavelength λ2Aout=1572 nm. The 1164 nm and 1572 nm light from
the amplifiers 106A, 106B is then provided to the first stage of the
frequency converter 110. In this embodiment, the frequency converter 110
includes two LBO crystals 111A, 110B, and two CLBO crystals 110C, 110D.
The first stage of the frequency converter 110 corresponds to the LBO
crystal 110A. The 1164 nm and 1572 nm light beams are sum frequency mixed
(SFM) in the LBO crystal 110A to produce the 634.5 nm light. The 634.5 nm
light then passes through the dichroic mirror M1, which filters out the
residual 1064 um light. The residual (10% to 90%, preferably 40% in this
embodiment) 1064 nm light is then directed towards the third nonlinear
crystal, CLBO 110C. The second LBO crystal 110B converts (via second
harmonic generation, SHG) the 634.5 nm to the 317.3 nm light which is
reflected by the dichroic mirror M2 and toward the CLBO crystal 110C. The
1064 nm and 317.3 nm light beams are sum frequency mixed (SFM) in the
CLBO crystal 110C to produce the 244.4 nm light. The dichroic mirror M3
filters out the residual 317 nm light, but passes through the 244.4 nm
light and the residual 1064 nm light (10% to 90% of light incident on the
CLBO crystal 110C, preferably 50% in this embodiment) remaining unused
after passing through the nonlinear crystals 110A and 110C. The 1064 nm
and 244.4 nm light beams are sum frequency mixed (SFM) in the CLBO
crystal 110D to produce the output 198.7 nm light.

[0044]The output power Pout of the output wavelength
λout=198.7 nm is proportional to the product of the two input
powers Pout˜P1064 nm×P244.4 nm, where
P1064 nm is the optical power (e.g. 20 W) of 1064 nm (IR wavelength)
provided to the CLBO crystal 110D and P244.4 nm is the 244.4 nm (UV
wavelength) optical power provided to the CLBO crystal 110D from the
third stage CLBO crystal 110C. Because the damage to the nonlinear
crystals is primarily caused by the high power beams in short wavelengths
range (UV), we can provide less optical power in the UV range from the
CLBO crystal 110C, and more optical power from longer wavelength source
(P1064 nm), thereby "trading" short wavelength (UV) incident power
for long wavelength (IR) incident power, and thus avoiding or minimizing
potential crystal damage. Similar "trade" was performed to prevent damage
to the CLBO crystal 110C by the 317.3 nm light. Accordingly, it is
preferable that laser that provides longer wavelength light to the last
stage of the frequency converter 110 (but it can be provided to any SFM
stage), such as, in this embodiment, both the Yb-doped fiber amplifier
106A and the Er-doped fiber amplifier 106B, provide the output optical
power of at least 10 W, and preferably at least 50 W to the frequency
converter 110.

[0045]The optimum temperature for the first LBO crystal 110A is 287.5
Kelvin. At this temperature, the crystal operates in non-critical phase
matching (for light propagating at the angles of θ=90° and
φ=0° to the optic axes of the crystal) with the effective
nonlinearity coefficient of deff=0.83 pm/V and essentially zero
birefringent walk-off for the sum frequency mixing of 1064 and 1572 nm.
The second LBO crystal 110B can not be non-critically phase matched. For
the light propagating at θ=90° and φ=54.5° to the
optic axes, the phase matching temperature for the second harmonic
generation of 634.5 nm is 433 K, the effective nonlinearity is
deff=0.53 pm/V and the birefringent walk-off is 17.05 milliradians.
The CLBO crystals 110C and 110D are uni-axial. For the third non linear
crystal, CLBO 110C light is propagating at θ=560 to its optic axis,
the phase-matching crystal temperature is 433 K, the effective
nonlinearity is deff=0.78 pm/V and the birefringent walk-off is
37.25 milliradians, for the nonlinear process of sum frequency mixing of
1064 nm and 317.26 nm light. For the fourth crystal, CLBO 110D, for light
propagating at θ=81.3° to its optic axis, the phase-matching
crystal temperature is 433 K, the effective nonlinearity is
deff=1.07 pm/V and the birefringent walk-off is 12.99 milliradians,
for the nonlinear process of sum frequency mixing of 1064 nm and 244.4 nm
light. As shown in FIG. 3, no OPOs were utilized in this embodiment of
the laser system 10. Table III provides the summary of crystal's
parameters utilized in the laser system 10 of example 3.

[0046]Table III provides the summary of crystal's parameters utilized in
the laser system 10 of example 3.

[0047]It is noted that in the above examples the embodiments of laser
system 10 do not utilize OPOs, thus producing stable outputs at the
desired output wavelength. The output wavelength λout is
uniquely determined by the wavelengths of lasers(s)/amplifier(s) 106A,
106B of the light source 102 and is not dependent on phase matching to
keep it stable.

[0048]It will be apparent to those skilled in the art that various
modifications and variations can be made to the present invention without
departing from the spirit and scope of the invention. Thus it is intended
that the present invention cover the modifications and variations of this
invention provided they come within the scope of the appended claims and
their equivalents.